[IPSEC]: Check x->encap before dereferencing it

We need to dereference x->encap before dereferencing it for encap_type.
If it's absent then the encap_type is zero.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>